The present invention relates to hot melt adhesives, and more particularly to a water sensitive hot melt adhesive composition based on alkyleneimine copolymers.
The term water sensitive hot melt adhesive refers to a category of adhesives including, water soluble, water dispersible, water releasable and repulpable hot melt adhesives. The adhesive of the present invention particularly relates to water soluble and water dispersible products which can be disintegrated by the action of water, thereby permitting paper products, disposable nonwoven articles, tapes and labels, and other disposable products to be more effectively recycled. The adhesives of the present invention can be pressure sensitive or non-pressure sensitive. The present invention is also directed to water sensitive adhesives that are particularly useful in the assembly of disposable nonwoven articles.
Hot melt adhesives typically exist as solid mass at ambient temperature and can be converted to flowable liquid state by the application of heat. These adhesives are particularly useful in manufacturing of a variety of disposable goods where bonding of various substrates is often necessary. Specific applications have included disposable diapers, sanitary napkins, pantyshields, surgical drapes, hospital pads and adult incontinence briefs; collectively known as disposable nonwoven products. Other diversified applications have involved paper products, packaging materials, tapes and labels. In these applications, the hot melt adhesive is heated to the molten state and then applied to a substrate. A second substrate is immediately laminated to the first and the adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesives is the lack of a liquid carrier, as would be the case for water-based or solvent based adhesives, thereby eliminating the costly drying step during application. Suitable hot melt adhesives must possess the appropriate bond strength to adhere the substrates involved, and must also possess adequate flexibility, no staining or bleedthrough, suitable viscosity and open time to function on commercial equipment, acceptable stability under storage conditions, and acceptable thermal stability under normal application temperature.
Many different polymers have been used in hot melt adhesives employed in the construction of disposable goods. In this regard, typical hot melt adhesives have employed polymers which have included styrene-isoprene-styrene (SIS); styrene-butadiene-styrene (SBS); styrene-ethylene-butylene-styrene (SEBS); ethylene-vinyl acetate (EVA); and amorphous poly-alpha-olefin (APAO). While these polymers, when properly blended, provide acceptable adhesion between most substrates employed in typical disposable goods construction such as diapers or packaging materials, they have had several shortcomings which have detracted from their usefulness.
One of the most noteworthy shortcomings of the prior hot melt adhesives concerns their solubility in water. Typical hot melt adhesives are insoluble in water and thus difficult to recycle. It is very desirable to recycle paper products and other disposable goods to conserve raw material resources and landfill space. It is thus a general practice in the paper industry to collect and repulp the waste paper to recover the cellulose fiber and to regenerate other useful paper products such as toilet paper or paperboard. Since no readily convenient method is currently available for recycling, or otherwise separating the components of disposable goods such as disposable diapers into component parts whereby the component parts may be recycled or alternatively disposed of in an environmentally friendly manner, to reduce any adverse impact these objects may have on the environment, concerns have frequently been raised regarding the environmental consequence of such products once they become unserviceable and disposed of in landfills or the like. The insolubility of the conventional hot melt adhesives further exacerbates such recycling problems. Thus, it would be desirable to utilize water sensitive hot melt adhesives to help solve this disposal and recycling problem since these adhesives will lose their bonding ability by soaking in water, thereby permitting the bonded object to disassemble into its component parts for easy separation and recovery. However, it must be understand that any such water sensitive hot melt adhesive must perform its intended function during storage and service where contact with moisture is, in many cases, unavoidable. As a result, the water sensitivity of hot melt adhesives must be balanced against the need to provide adequate bond strength to maintain the structural integrity of the products until disposal.
The most common water sensitive or water soluble hot melt adhesives are based on polymers containing vinyl pyrrolidone or other vinyl heterocyclic monomers as taught by Colon et al, in U.S. Pat. Nos. 4,331,576 and 4,325,851. These adhesives are made from vinyl pyrrolidone polymers, vinyl pyrrolidone/vinyl acetate copolymers, and other vinyl pyrrolidone based polymers. Another class of water sensitive hot melt adhesives include the polyester based adhesives comprising a single or a blend of copolyesters in combination with a plasticizer, which are disclosed in Morrison, U.S. Pat. No. 4,052,368, and in Harrington, Jr., et al, U.S. Pat. No. 4,172,824, and in Miller, et al, U.S. Pat. Nos. 5,543,488, 5,552,511 and 5,605,764.
The polyvinyl pyrrolidone and polyester based adhesives are typically applied as a liquid to a substrate from a reservoir or glue pot heated to temperatures between 150.degree. C.-200.degree. C. Water sensitive adhesives based on a polyvinyl pyrrolidone or a polyvinylpyrrolidone/vinyl acetate exhibit poor thermal stability. Such adhesive formulations can rapidly degrade at the elevated pot temperatures which are employed during their application. Copolyester based adhesives, due to the potential chemical reactivity of copolyester with many other ingredients that are often used to make a useful hot melt adhesive, suffer from limitations in formulation latitude and therefore from the lack of balance between water sensitivity and bond strength, thereby further limiting their usefulness.
Other water sensitive hot melt adhesives are based on polyvinylmethylether. They contain either water soluble or insoluble plasticizers and a large amount of water insoluble tackifier. Polyvinylmethylether has limited solubility in water which limits its utility in many applications. Also, the polymer has poor thermal stability, has a dark color raising aesthetic concerns, and has a strong objectionable odor problem.
Finally, water soluble hot melt adhesives based on substituted polyalkyleneimine (polyoxazoline) homopolymers are disclosed in Scholl et al U.S. Pat. No. 5,143,961 and Bunnelle et al U.S. Pat. No. 5,459,184. Such adhesives, however, are brittle and exhibit limited cohesive strength. In addition, such adhesives usually have high viscosity which often limit their applications in disposable goods.
Accordingly, a need exists for a hot melt adhesive composition that is substantially heat stable, water sensitive, can form strong bonds, can be formulated to be pressure sensitive.